Exhaust gas exhausted from internal combustion engines of vehicles such as buses, trucks or construction machines contains particulate and noxious gas having a noxious influence on environments and human bodies. Therefore, in recent years, there is proposed techniques for collecting and removing the particulate from exhaust gas. One of these techniques provides a ceramic structural body used as a filter for purifying the exhaust gas. This ceramic structural body comprises a honeycomb structural porous ceramic member, and embodied as a ceramic honeycomb filter for collecting and removing particulates from exhaust gas.
The ceramic filter uses, as shown in FIG. 1A, a ceramic honeycomb structural body 10 which is formed by assembling and integrating a plurality of rectangular pillar-shaped porous ceramic members (units) 10 with each other by interposing seal layers 14 as a ceramic block and the outer surface of the ceramic block is covered with seal layers 16 for preventing exhaust gas from leakage. This ceramic honeycomb structural body 10 makes the ceramic member (unit) 11 having the pillar-shaped honeycomb structure as one construction unit and these plural members are bound together to be formed into circular, oval or polygonal shape in section.
As shown in FIG. 1A, in respective ceramic members 11 are provided numbers of cells 12 as exhaust gas passages through the cell walls 13. And, in case of using the ceramic honeycomb structural body 10 as an exhaust gas purifying filter, ends of the cells 12 are mainly plugged alternately with plugging for collecting particulates in exhaust gas on the cell walls 13 when the exhaust gas flowed into the cells 12 passes (wall flow) through the cell walls 13.
As described above, the reason of constructing the ceramic honeycomb structural body (filter) by binding a plurality of pillar-shaped ceramic members is as follows. That is, when a ceramic honeycomb structural body is made large by integrally molding, even if silicon carbide having high heat resistance, excellent mechanical characteristics and large thermal conductivity is used as a ceramic member, temperature difference is generated in the ceramic honeycomb structural body by temperature change and local heat generation of exhaust gas, and the ceramic honeycomb structural body tends to be cracked by thermal expansion difference due to this temperature difference. Therefore, this filter is made by assembling a plurality of ceramic members (units), and respective ceramic members (units) are adhered to each other by interposing seal layers to form an assembly to relax thermal stress acted on the filter.
Then, there has been proposed in Japanese unexamined patent publication No. 2001-138416 (JP-A-2001-138416) that unevenness or waviness of about 0.2 mm to 1.5 mm in flatness is given to the outer wall as a joining face of each honeycomb segment (ceramic member).
The contents of JP-A-2001-138416 are incorporated herein by reference in its entirety.